This invention relates in general to an improved surface wave communication system particularly useful in providing communication between moving objects. More specifically, the invention relates to means for coupling radio frequency signals to and from a single wire transmission line to provide a wide bandwidth communications link between relatively moving transmitters and receivers, wherein the signals travel along the transmission line in the form of surface waves.
Surface wave communication systems are generally known in the art and have occasionally been used for limited purposes. Generally, however, when it has been necessary to provide a communication link between two points or objects which are not fixed, other types of communication systems such as radio transmission have been used. An obvious drawback to the conventional radio transmitter and receiver method of communication is the ease with which transmitter information can be monitored or jammed. In addition, radio transmission is dependent upon terrain and atmospheric conditions and often receivers only a short distance away from a transmitter are unable to receive a transmitted radio signal due to blockage of the signal by natural barriers, such as mountains, electrical storms, and other phenomena which can totally obliterate a transmitted radio wave. A type of communication systems which is free of these problems and thus can be relied upon for accurate and dependable transmission of signals has obvious advantages. For these and other reasons, transmission by wire often is a preferred alternative to radio, or wireless, transmission.
Of course, direct wire communication systems such as the conventional telephone system are widely used to provide a private transfer of information, and such systems have many of the advantages alluded to above. However, telephone systems presently available require a direct connection of the transmission line to the transmitter at one end and to the receiver at the other, which inhibits the mobility of this type of system, and makes it unsuitable for communication between relatively moving transmitting and receiving stations.
Because of the signal attenuation inherent in a transmission line, it is desirable for maximum signal strength and clarity that the length of line between a transmitter and receiver be kept to a minimum. However, this is not generally feasible where the distance between the transmitting and receiving stations varies, and the usual practice is to connect the transmitter and receiver to opposite ends of a line having a length equal to the maximum distance between the stations, thus producing maximum signal attenuation at all times. Where an excess amount of line is provided, however, the extra length must be stored in some manner, while preserving easy access when the distance between the stations changes, and accordingly the line is usually wound on suitable storage reels. This in itself produces a further problem because the coiling of the transmission line produces a change in line impedance which affects the transmission of signals. A reduction in the length of the line, and thus in signal attenuation, could be obtained if the connection points of the receiver, the transmitter, or both, to the line could be changed as the length of line required to span the distance between the stations changed. However, with conventional couplers, such changes are time consuming and difficult, and thus not suitable for situations where the transmission line length must be changed frequently or where the changes are very rapid.
The present invention overcomes the foregoing problems through the provision of a coupler which is coaxial to the transmission line and through which the line can pass with only a minimum effect on the coupling of signal energy to or from the line.